1. Field of the Invention
The present invention relates generally to the field of integrated circuit design, and specifically, to multilevel analog signal recording and playback systems.
2. Background Information
The ISD 4000 and the ISD 33000 series products of Information Storage Devices, located in San Jose, Calif., represent a 3-volt serial interface family of voice recording and playback devices. These devices only record messages at a single sample rate, giving a fixed duration product. These prior art devices further include singular, fixed interface capability, i.e., a single input of a specified level and a single output of a specified level. In many systems, there are more than one signal source and/or destination. Specifically, in mobile communication systems, signals flow to and from the human interface (microphone and speaker), the baseband processing module, and other interfaces. To provide such a system, many external components are required in order to switch and/or mix signals, and adjust signal levels.
FIG. 1 illustrates a block diagram of a mobile communication system 10 incorporating a prior art analog recording and playback system. The system 10 includes a baseband section 20 that is coupled to a radio frequency section 22, which is in turn coupled to an antenna 24 for transmitting and receiving signals. The baseband section 20 includes a digital signal processor 26 for performing conventional signal processing, a baseband codec 28, and a voiceband codec 30. The voiceband codec is coupled to a microphone 32 and a speaker 34, both of which may be incorporated into a handset, through microphone (MIC+ and MIC−) and speaker (SP OUT+ and SP OUT−) terminals, respectively. The system 40 is coupled to the baseband section 20 for recording messages (memos, or a message from a remote caller) by way of an ANA IN input, and for playing back messages (outgoing messages) by way of the ANA OUT output. A microcontroller 50 is coupled to the baseband section 20 and system 40 for controlling both devices.
As can be seen, the prior art system of FIG. 1 together with other products were designed solely to record or playback an analog signal. They contain no analog processing functions such as signal mixing, gain, or level control. This implies that such functions must be provided by external components, often making implementation of such functions impracticable due to economic and/or physical space constraints.